Fabric Energy Efficiency First

DFEE v TFEE Assessment is an Essential Part of Planning Submissions

Issues are increasingly arising from dwellings that are designed at early Planning Stage to comply with Local Authority Planning Renewable Requirements and meet Part L1a Criterion 1 DER v TER (CO2 Emission Rates) whose fabric and heating system meet Part L1a criterion 2, and are of low risk of summertime overheating and thus meet Part L1a Criterion 3 but actually fail on Criterion 1 DFEE v TFEE (Fabric Energy Efficiency Rates) and thus fail overall because of a dependence on LZC technology (e.g. PV) to meet DER v TER criterion.

Such failures can be hidden during the planning process and may prove costly to reconfigure at a later stage.

NOTE: Criterion 1 is a regulation and therefore mandatory; other criteria are not necessarily mandatory they are considered statutory guidance.

Example – Detached Dwelling

The example given is a detached dwelling built over a car port. The Fabric details and specifications are outlined below. The dwelling is west facing and PV is to be installed on the east facing roof.


Fabric (W/m2K)

U Floor 0.14
U Walls 0.15
U Roof 0.13
U Windows 0.8
Y Thermal Bridging 0.15
Perm m3/(h.m2) 6
Boiler 89.5%
PV (kWp) 1.3


kg CO2/m2/year
DER 22.46
TER 25.35
kWh /m2/year

Energy Evaluation Comment:

As can be seen all the fabric and heating specification readily complies with Part L1a Criterion 2 and the DER emissions are projected to be more than 10% below the TER thus meeting Local Planning criteria.

However as can be seen the DFEE is > TFEE and not compliant and the dwelling fails over all.

The target TFEE rate is based on the provision and standardised use of specified fabric and fixed building services, the TFEE target cannot be offset through the use of LZC technology.


In this case the TFEE target could be met through increased attention to thermal bridging and the use of Approved Details. However this would require consideration at an early stage of the design process.

For further information or advice please contact one of Energy Evaluation consultants

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